In the past a tremendous amount of work has been devoted to the development of particles, in particular catalytically active particles, for many different processes. There has also been a considerable effort to try to understand the advantages and sometimes disadvantages of effects of shape when deviating from conventional shapes such as pellets, rods, spheres and cylinders for use in catalytic as well as non-catalytic duties.
Examples of further well-known shapes are rings, cloverleafs, dumbells and C-shaped particles. Considerable efforts have been devoted to the so-called “polylobal”-shaped particles. Many commercial catalysts are available in TL (Trilobe) or QL (Quadrulobe) form. They serve as alternatives to the conventional cylindrical shape and often provide advantages because of their increased surface-to-volume ratio which enables the exposure of more catalytic sites thus providing more active catalysts.
An example of a study directed to effects of different shapes on catalytic performance can be found in the article by I. Naka and A. de Bruijn (J. Japan Petrol. Inst., Vol. 23, No. 4, 1980, pages 268–273), entitled “Hydrodesulphurisation Activity of Catalysts with Non-Cylindrical Shape”. In this article experiments have been described in which non-cylindrical extrudates with cross-sections of symmetrical quadrulobes, asymmetrical quadrulobes and trilobes as well as cylindrical extrudates with nominal diameters of 1/32, 1/16 and 1/12 inch were tested in a small bench scale unit on their hydrodesulphurisation activity (12% wt MoO3 and 4% wt CoO on gamma alumina). It is concluded in this article that the HDS activity is strongly correlated with the geometrical volume-to-surface ratio of the catalyst particles but independent of catalyst shape.
In EP-A-220933, published in 1987, it is described that the shape of quadrulobe-type catalysts is important, in particular with respect to a phenomenon known as pressure drop. From the experimental evidence provided it appears that asymmetric quadrulobes suffer less from pressure drop than the closely related symmetrical quadrulobes. The asymmetrically shaped particles are described in EP-A-220933 by way of each pair of protrusions being separated by a channel which is narrower than the protrusions to prevent entry thereinto by the protrusions of an adjacent particle. It is taught in EP-A-220933 that the shape of the particles prevents them from “packing” in a bed causing the overall bulk density of the catalyst bed to be low.
Since many of the findings in the art are conflicting and pressure drop problems continue to be in existence, especially when surface-to-volume ratios are increased by reducing particle size, there is still considerable room to search for alternative shapes of (optionally catalytically active) particles which would diminish or even prevent such problems. It has now surprisingly been found that specifically shaped particles of the general “trilobal” shape offer unexpected and sizeable advantages compared with conventional “trilobal” particles, both in catalytic and non-catalytic duty.